EN
We study the class of all rearrangement-invariant ( = r.i.) function spaces E on [0,1] such that there exists 0 < q < 1 for which $∥∑_{k=1}^{n}ξ_{k}∥_{E} ≤ Cn^{q}$, where ${ξ_{k}}_{k≥1} ⊂ E$ is an arbitrary sequence of independent identically distributed symmetric random variables on [0,1] and C > 0 does not depend on n. We completely characterize all Lorentz spaces having this property and complement classical results of Rodin and Semenov for Orlicz spaces $exp(L_{p})$, p ≥ 1. We further apply our results to the study of Banach-Saks index sets in r.i. spaces.